Pro-Russia activists set their flags over the entrance to the regional administration building in Donetsk, Ukraine, on April 8. / Efrem Lukatsky, AP

by Olga Rudenko , Special for USA TODAY

by Olga Rudenko , Special for USA TODAY

KIEV, Ukraine - Ukrainian authorities warned that they will use force by Friday to clear out several government buildings seized by pro-Russia separatists in the east of the country as the United States blamed Moscow for stirring up the unrest.

Armed pro-Russia separatists released 56 hostages taken overnight Wednesday in the eastern city of Luhansk after a government move to shore up Ukraine's territorial integrity as Russian troops loom on the border.

Interior Minister Arsen Avakov said the standoff in Luhansk and the two neighboring Russian-leaning regions of Donetsk and Kharkiv must be resolved within the next two days.

"I want to repeat that there are two options: political settlement through negotiations and the use of force," Avakov said. "We are ready for both options."

Secretary of State John Kerry said the United States will push for tougher economic sanctions if Russia refuses to end its attempt to cause insurrection in Ukraine.

Tuesday, Kerry said Russia was attempting an "illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis with paid operatives across an international boundary." Ukraine fears Russia will use the unrest as a pretext for an invasion as it did in the Ukrainian province of Crimea, which is in Russian military hands.

On Wednesday, the Russian Foreign Ministry accused the United States of using international organizations as a means of "exacerbating tensions surrounding Ukraine."

"The daily activity of Russian troops on national territory does not threaten the security of the U.S. or other member states of the OSCE (Organization for Security and Cooperation in Europe)," the Russian statement said. "Attempts to accuse Russia of a buildup of troops are unfounded."

About 1,000 separatists held the Luhansk branch of the State Security Service after seizing it Sunday. They released 51 hostages Wednesday and five more four hours later, according to the State Security Service, which handled negotiations. It is unclear whether there are more hostages in the occupied building.

Since Sunday, other groups took over several administrative buildings in eastern cities, including Donetsk, close to the Russian border. The groups demand that the eastern regions hold referendums on joining Russia and have asked Russian officials to send troops in to support a vote.

Negotiations continue over two buildings in Luhansk and Donetsk that remain under control of separatists.

Some say they are disappointed by the lack of more decisive action on the part of police, saying they are not doing enough.

"I understand that police and government have to act appropriately as to not give Russia a cause to send in troops," said Viktoria Melnyk of Luhansk. "But these events hit really hard on the nerves of people here. It is not normal to let a bunch of armed guys seize buildings and hold them for days."

In response to the escalation in the east, Ukraine's government adopted a law increasing punishment for crimes against Ukraine's integrity. Those accused of "separatism" can get up to 15 years in prison, or life imprisonment if their actions result in casualties.

"The law is really useful in the current situation although it seems like they should've adopted something like this earlier," Melnyk said. "The separatist movements didn't start this Sunday; in the east, they were going on for weeks."

The State Security Service arrested Maria Koleda, 23, who allegedly encouraged pro-Russian movements and organized street clashes in the southern cities of Ukraine.

Serhiy Tyhipko, a lawmaker associated with the previous Ukrainian government who is now in opposition, said he was able to enter the seized building in Luhansk on Tuesday night, and he said there were no hostages inside. He urged Kiev to take note of the protesters' demands.

"The authorities are not listening to the southeast. People are putting forward certain demands, but nobody hears (them)," Tyhipko was quoted as saying by the Interfax news agency.

All the cities affected by the uprisings are in Ukraine's industrial heartland in the east, which has a large population of ethnic Russians and strong economic and cultural ties to Russia. Many residents are suspicious of the government that took power in February after ousting pro-Moscow President Viktor Yanykovych, whose forces shot down more than 80 protesters in demonstrations in Kiev.

Overnight, speakers at a gathering in front of the building in Luhansk condemned the government in Kiev and renewed demands to be allowed to hold a referendum on declaring autonomy for their region. That demand is similar to one that preceded Crimea's annexation by Russia.

Up to 40,000 Russian troops are massed along the Ukrainian border, according to NATO.

Those occupying the building issued a video statement warning that any attempt to storm the place would be met with force. In the video, posted by Ukrainian media, a masked man identified the occupiers as Ukrainian veterans of the Soviet war in Afghanistan and warned the authorities against trying to retake the building. "Welcome to hell, then!" he said.

Parliamentary deputies were able to enter and leave the building unhindered.

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